"No man ever steps in the same river twice, for it's not the same river and he's not the same man." — Heraclitus, lived around 500 BC in Ephesus, modern-day Turkey.

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Like many badass thinkers, Heraclitus of Ephesus was born wealthy in a city but lived in the woods to contemplate the universe.

About 2300 years ago, he had an insight that would reverberate down intellectual history — that the universe is in a constant state of flux. And as the above quote asserts, so are our very identities.

The sage "is ready to use all situations and doesn't waste anything. This is called embodying the light." — Lao Tzu, alive around 600 BC in China.

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The Lao Tzu started Taoism in 2500 years ago in China. Scholars say he's "semi-legendary," since Lao Tzu just means "Old Man" and nobody knows who he actually was.

More importantly, he left us the inscrutably playful text the "Tao de Cheng." It's full of zingers, such as the above observation, which basically says every situation you find yourself in, no matter how bad it seems, is fundamentally workable.

"Suffering exists." — Siddhartha Gautama, lived around 500 BC in the plain of the Ganges, modern India.

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Often mistranslated as the much less hopeful lifeis suffering, Siddartha Gautama's observation that suffering simply exists is the bedrock of the religion built around his teachings — Buddhism.

When you recognize that suffering exists, it's a lot easier to avoid getting freaked out by it.

"To rank the effort above the prize may be called love." Confucius, alive in China around 500 BC.

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Confucius is probably the most influential person in Chinese history.

The country — and the entire East Asian region's — emphasis on familial relationships and duty to the state can be traced back to this sage.

He emphasized what we today may call grit: finding the value in trying to achieve, not attaining actual achievements.

"The unexamined life is not worth living." — Socrates, lived in Athens around 450 BC.

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Socrates embodied the fundamental spirit of Western thought: that you,yes you, have the responsibility of being the author of your own life.

To do that, the Ancient Greek said, you must examine.

"The life of money-making is one undertaken under compulsion, and wealth is evidently not the good we are seeking; for it is merely useful and for the sake of something else." Aristotle, alive in Ancient Greece around 300 BC.

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Not only was Aristotle one of the first people to lay out ideas about the natural world that we might now call scientific, not only did he lay down some of the best rules for writing, but the guy also gave a critique of living a capitalistic life 2000 years before Karl Marx hit the scene.

Living for money wasn't a good call, Aristotle observed, since money was only useful when leveraged as a tool to gain something else — like security or status.

"When you are inspired by some great purpose, some extraordinary project, all your thoughts break their bonds." Patanjali, alive in India around 100 BC.

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Patanjali put together the yoga sutras, the philosophy underlying the sun salutations you might start the day with.

The above quote continues:

"Your mind transcends limitations, your consciousness expands in every direction, and you find yourself in a new, great, and wonderful world," he writes. "Dormant forces, faculties and talents become alive, and your discover yourself to be a greater person by far than you ever dreamed yourself to be."

Pretty epic.

"Although you may spend your life killing, you will not exhaust all your foes. But if you quell your own anger, your real enemy will be slain." Nagarjuna, India, alive around 200 CE.

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Nagarjuna was probably the most important Buddhist philosopher since Siddartha Gautama. The above quote speaks to his emphasis on being intimate with one's own interior world.

He was a rigorous logician who argued that any statement you make can be in some way falsified — the takeaway being that if you're looking for truth, it's probably not going to come in words.

Rather, it will be through direct experience. Like — as you may expect from a Buddhist saint — through meditation.

"Most powerful is he who has himself in his own power." — Seneca, died 65 AD in Rome.

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The Roman Seneca was a stoic philosopher and an advisor of the emperor Nero.

Seneca anticipated what psychologists today call 'locus of control.' People with an external locus of control think that the events in their life stem from factors outside themselves, like fate or a deity. People with aninternal locus of control think they are in charge of the events in their lives, and are more likely to turn the lemons of life's tragedies into the lemonade of wisdom — which is precisely what Seneca exhorted the reader to do.